There is in the offshore drilling, production, and transportation industry a variety of structures for supporting men and machinery at stations offshore. The structures are similar in basic function, namely to support the men and machinery in accomplishing their assigned function. Otherwise the structures are significantly different in that some are mobile while others are stationary; and some are founded on the marine floor while others float. The mobile structures are commonly called "mobile rigs" while the fixed ones are "fixed platforms". These rigs and platforms are generally classified as either drill ships, semi-submersibles, submersibles, jackups, pile jackets, or one of a few other hybrid designs. Each is peculiarly designed and adapted for a fairly limited set of operational conditions. As a result, there is restrictive flexibility of use. For example, the fixed frame design, commonly known as the aforementioned pile jacket, is best suited for permanent stations in water of a few hundred feet or less. All other current types are mobile and more expensive as a result, and therefore they are well suited for exploration work, but not so well suited for long duration stationary production work. However, even production work is not truly permanent and reusability would be a significant consideration in the design of a rig for production purposes. Fixed platforms are seldom used in exploration work because the cost of building a new platform for each exploratory hole is almost prohibitive, except in shallow, protected waters. Thus fixed platforms are primarily production type structures.
In contrast to the permanent station characterizing the fixed frame design, the most mobile type of offshore platform is the drill ship with its appropriate stabilizing and stationing apparatus such as thrusters, anchors, and winches. The ship certainly is not best suited for permanent stationing because it is particularly sensitive to wave action by reason of the wide surface area of structure exposed to the water, and the use of thrusters to maintain stationing for long periods of time, as would be necessary for production activities, is economically unfeasible because of fuel consumption and the necessarily continuous human monitoring which is required. Also, of course, the ship is vulnerable to storms and generally adverse conditions.
Intermediate the permanent station characterizing the fixed frame and the high mobility of the drill ship are the family of rigs known as jackups. The jackup rig has some of the advantages of both the fixed frame and the drill ship. It is analogous to the former in that it is supported on the marine floor, and therefore is quite stable and requires no continuous operation or monitoring to maintain position. And, when the legs are raised, it can be towed from place to place so as to thereby provide a degree of mobility analogous to the drill ship. The legs can, of course, be lowered to varying elevations so as to provide the jackup with an adaptability to a variety of depths.
The aforementioned submersibles are also an intermediate form of rig. The submersibles and also the semi-submersibles have good stability, each is suited to its depth, the semi-submersible to deep water, the submersible bottom-founded frame to shallow water. Both are more stable but less mobile than the drill ship. They cannot be moved in heavy weather without jeopardy to themselves, their crew, and the towing vessel; and indeed some have even been lost while being moved in calm waters.
It thus becomes evident without further elaboration that each of the various types of offshore drilling rigs commonly used is well suited only to a relatively narrow range of operation. As a result, a company or firm which chooses to operate offshore must elect the particular type of rig best suited for its contemplated initial endeavor and thereafter be committed to the limitations of that rig when it is used in subsequent drilling operations.
The limitations inherent in the state of the art are best illustrated by a brief examination of one of the more common types of mobile rig. The jackup rig, as previously noted, has certain of the advantageous characteristics of the pile jacket in that it is founded on the bottom in a stable way while the apparatus it carries is supported clear of the highest anticipated waves so that the whole is a fairly stable semi-permanent, albeit expensive, station. Mobility is achieved by lowering its bouyant platform to the water, and raising the legs from the marine floor, so that the whole can then be floatably moved to another location. Although jackup rigs in general are adapted to a relatively wide range of operating depths, namely about 20 to perhaps 500 feet, any single jackup is adapted, as a practical matter, to a very limited subrange within this range. For example, if a jackup of the prior art were to be designed for work in the Gulf of Mexico in water of 200 foot depth, it certainly could not work in depths of 400 feet because its legs would not reach the bottom. Likewise, it could not work competitively, that is economically, in water depths of say 60 feet because other less expensive jackup rigs would be available for that work. So, although in a physical sense jackup rigs can work in a wide range of depths, they cannot in an individual and in an economic sense encompass a very large range of usefulness.
Therefore, it might be concluded from the above brief discussion that the offshore operator is restricted to a fixed location rig which represents a substantial investment, or to a mobile rig which represents a very substantial investment but which is adapted only to a relatively narrow range of operational depths which may represent a certain class of locations.